Bridge rectifier circuits are commonly used to convert alternating-current (AC) voltages to direct-current (DC) voltages, and may be designed to convert poly-phase AC voltages, such as the three-phase AC voltages commonly produced in power generation systems, into a single DC voltage, with low losses.
The simplest bridge rectifier circuits utilize diodes as the switching elements. However, these circuits are not controllable, in that the circuits do not allow regulation of their DC output voltages. Controllable bridge rectifier circuits thus use controllable devices, such as thyristors, insulated-gate bipolar transistors (IGBTs), or integrated gate-commutated thyristors (IGCTs). One common configuration for a controllable bridge rectifier circuit is shown in FIG. 1, which illustrates a six-pulse, controlled, bridge rectifier circuit connected to a three-phase AC source having commutating inductances in each phase.
In the controllable bridge rectifier circuit of FIG. 1, thyristors D1-D6 are arranged in three pairs, where each pair is connected in series. The midpoint between each pair is connected to one of the phases of the AC sources. In the discussion that follows, a pair of devices arranged in this manner is referred to as a “phase leg,” while the upper and lower halves of each phase leg is referred to as an “arm.” Thus, thyristors D1 and D4 together form a phase leg in the circuit of FIG. 1, while each of the thyristors individually corresponds to an arm of that leg. A second phase leg comprises thyristors D3 and D6, while a third phase leg includes thyristors D2 and D5. Corresponding endpoints of each phase leg are connected together, and the DC output Vdc is taken from across these endpoints, as seen in FIG. 1.
The operation of a controllable bridge rectifier circuit like the one shown in FIG. 1 is well known. For an ideal circuit (i.e., ignoring the commutating inductances shown in FIG. 1), the no-load average output voltage is
            V      dc        =                  V        ac            =                                    3            ⁢                          3                        ⁢                          V              peak                                π                ⁢        cos        ⁢                                  ⁢        α              ,where Vpeak is the peak value of the phase (line-to-neutral) input voltages and a is the firing angle of the thyristors, i.e., the phase angle, relative to the zero-crossing point of the AC waveform in a given thyristor's phase leg, at which the thyristor is triggered into conduction. If the commutating inductances are considered and assumed to have an inductance of Ld, then the output voltage is a function of the DC load current Id, and is given by
            V      dc        =                  V        ac            =                                                  3              ⁢                              3                            ⁢                              V                peak                                      π                    ⁢          cos          ⁢                                          ⁢          α                -                  6          ⁢                                          ⁢                      fL            d                    ⁢                      I            d                                ,where f is the AC frequency.
Insulated-gate bipolar transistors (IGBTs) are also commonly used in controllable bridge rectifier circuits, especially in applications having higher switching frequencies (e.g., above 1 kHz). FIG. 2 illustrates an example of such a circuit, in which IGBTs T1-T6 replace the thyristors D1-D6 of FIG. 1. The circuit of FIG. 2 is otherwise the same as that pictured in FIG. 1, except that the circuit of FIG. 2 includes AC source resistances as well as the commutating inductances.
Electric power distribution systems are increasingly incorporating high-voltage, direct-current (HVDC) transmission links. HVDC systems offer several advantages over conventional AC transmission—they can be less expensive and have lower losses for long transmissions, and they allow power transmission between unsynchronized AC systems.
In many applications, then, a low-cost, high-power rectifier system is needed to feed a HVDC bus or grid. These rectifier systems must be controllable, because the source voltage varies in amplitude (and may also vary in frequency) in some of these applications. For example, a typical wind or tidal generator output voltage ranges from 40-100% of the nominal output for the generator. A typical marine generator output voltage ranges from 70-100% of the nominal value. In these applications, typical nominal generator voltages may range from 690 VAC to 13.8 kVAC, while typical DC bus voltages may range from 10 kVDC to 120 kVDC. Low-cost solutions that provide a large degree of controllability in conjunction with high performance over a wide range of operating conditions are needed.